The present invention generally relates to transmission power control for satellite communication systems. In particular, the present invention relates to a method for calculating uplink transmission power adjustments necessary to compensate for environmental effects such as the effects of rain attenuation (also known as rain fade) and antenna pointing error in a satellite communication system.
Satellites have long been used to provide communications capabilities on a global scale. Typically, a modem communications satellite includes multiple uplink and downlink antennas, each of which provides communications bandwidth to a large coverage area (or "footprint") using multiple spot beams. Modem satellite antennas operate at much higher carrier frequencies than those in previous systems. Thus, for example, modem satellites may use Ka-band frequency uplinks (at approximately 30 GHz) and downlinks (at approximately 20 GHz), while previous satellites used Ku- or C-Band frequency uplinks and downlinks (approximately 3-12 GHz).
Ka-band frequency downlinks generate relatively small spot beams on the surface of the Earth. The area covered by a spot beam is commonly referred to as a cell. When spot beams are transmitted by a geosynchronous satellite, the diameter of an associated cell may be 300-400 miles. Generally, boundaries of a cell may be the points in the spot beam where the antenna gain falls to a minimally acceptable level, e.g., -5 or -6 dB, relative to the gain at the center of the spot beam. Thus, a large number of spot beams may be required to cover a large land mass.
Modem satellite communications systems operate with uplinks and downlinks at much higher carrier frequencies than earlier systems (for example, at Ka-Band). Consequently, modem satellite communications systems are susceptible to significant signal attenuation due to atmospheric conditions such as rain. Rain attenuation of the uplink and downlink represents a primary obstacle that must be overcome to achieve successful communication in the Ka-band. In the past, attempts to overcome rain attenuation problems typically have centered on transmission power control techniques. In general, conventional power control techniques attempt to adjust the transmission power of a ground terminal such that the power arriving at the satellite is constant, regardless of the rain attenuation affecting the uplink. Thus, when it rains and the attenuation due to rain increases, so does the ground terminal's transmission power. When the rain subsides, the ground terminal's transmission power is lowered commensurate with the current rain attenuation. Thus, conventional power control techniques are based on the magnitude of the rain attenuation.
Techniques have been proposed that attempt to correct for rain attenuation in order to maintain a constant level of received power at the satellite. As an example, Dissanayake, Asoka W., "Application of Open-Loop Uplink Power Control in Ka-Band Satellite Links," Proceedings of the IEEE, Vol. 85, No. 6, June 1997, pgs. 959-969, discloses a type of open-loop power control in which beacon signals are implemented and signal levels measured. The system described by Dissanayake includes a satellite beacon in the downlink signal. Dissanayake measures characteristics of the beacon received at an earth station. A corrective uplink power level is calculated based on the measured characteristics and based on knowledge of the effects of various atmospheric phenomena.
Another technique is disclosed in U.S. Pat. No. 4,910,792, which uses bent-pipe transponders on board the satellite to relay an uplink signal back to earth for measurement. Bent-pipe transponders relay uplink signals back to earth in downlink signals without any demodulation or remodulation. The '792 patent discloses a technique which compares two signals. One signal is a reference signal sent from a reference station to a satellite in an uplink which is then relayed to an earth station in a downlink, and the other signal is an uplink signal sent from the ground station to the satellite in an uplink which is then relayed back to the earth station in a downlink. The results of the signal comparison are then used to adjust uplink transmission power.
One drawback of bent-pipe techniques is that they cannot be directly implemented on more versatile decoding and switching satellite systems which do not include bent-pipe transponders. Another drawback is that imperfections in the uplink signal are included in the transponded downlink signal. Imperfections in the uplink signal render it difficult, if not impossible, to completely separate the atmospheric effects on the uplink from the atmospheric effects on the downlink.
However, conventional power control techniques have experienced certain disadvantages since they have failed to realize that rain attenuation is only one significant contributor to attenuation. Conventional power control techniques have failed to give special consideration to the effects of satellite antenna pointing error and the resultant shifts in spot beams and the overall antenna pattern. Antenna patterns are directional, and thus an incorrectly pointed antenna creates unexpected signal strength variations at the receiving mobile user or ground station. The affects of antenna pointing error are most pronounced near the boundaries of a spot beam or cell. In the past, most satellite communication systems were designed to cover relatively broad coverage areas (e.g., the United States). With a broad coverage area, the vast majority of terrestrial users are located well within the boundaries of the coverage area. Thus, the affects of relatively small satellite or antenna pointing errors are generally negligible. However, more recently proposed cellular satellite communications systems use spot beams which provide relatively small cells (e.g., covering a single city or region). Hence, the percentage of terrestrial users near the boundaries of the coverage area of each cell is no longer negligible. In addition, transmission power control in cellular communications systems assumes additional importance due to the high degree of frequency reuse and resulting cross-cell interference that occurs with improper transmission power control.
Past open-loop power control techniques generate significant errors in the estimate of the adjustment necessary in uplink transmission power when sources other than rain (particularly antenna pointing error) contribute to the downlink attenuation. As noted above, for ground terminals near the edge of the spot beam, antenna pointing errors may result in significant uplink and downlink attenuation. Misclassifying attenuation due to antenna pointing error as attenuation due to rain when estimating the uplink attenuation from the downlink attenuation can lead to significant errors, as explained below.
Atmospheric conditions, such as rain, may attenuate the uplink and downlink signals differently, depending on the uplink and downlink frequencies. For example, for communications signals in the Ka-band, the uplink attenuation due to rain may be approximately 2.25 times greater (when measured in decibels (dB)) than the downlink attenuation due to rain.
Atmospheric conditions, such as rain, may have uneven attenuation effects upon communications signals in the uplink and downlink because communications signals in the uplink have a different carrier frequency than signals in the downlink. For example, uplink signals may have a carrier frequency of approximately 30 GHz, while downlink signals may have a carrier frequency of approximately 20 GHz. The difference in carrier frequencies renders signals in the uplink much more susceptible to attenuation due to certain atmospheric conditions, such as rain, than signals in the downlink.
However, antenna pointing error may cause approximately the same amount of attenuation of communications signals in the uplink as in the downlink. An antenna has a predetermined gain pattern distributed across a given cell. As the antenna moves with respect to the cell, the antenna gain pattern across the cell similarly moves. Thus, when an antenna is pointed correctly, a mobile station or ground station in the corresponding cell is located at a known gain level along the gain pattern. However, when an antenna is misaligned, the associated antenna gain pattern is similarly misaligned with respect to the cell and to mobile stations or ground stations in the cell. Consequently, mobile or ground stations will experience attenuation (or amplification) due to a difference between the expected known gain level and an actual mis-aligned gain level. An antenna gain pattern may be designed to be roughly identical for two or more carrier frequencies. Therefore, signals in the uplink and in the downlink experience the same amount of attenuation due to antenna pointing error.
For example, a 5 dB loss may be measured in the downlink, and 3 dB of the loss may be due to antenna pointing error, and 2 dB of the loss due to rain. In conventional systems all of the attenuation would be attributed to rain, and therefore, a power correction for the uplink of 11.25 dB (2.25*5 dB) would be calculated. However, a more appropriate power correction would have been 7.5 dB (3 dB for antenna pointing error, and 2.25*2 dB for rain attenuation). Thus, past techniques overcompensate for attenuation. As a result, power is wasted, and cross-cell interference is increased unnecessarily.
A need exists for providing a method for determining appropriate uplink power correction under conditions of combined rain attenuation and antenna pointing error.